Re: [PATCH v4 2/4] hugetlb/userfaultfd: Forbid huge pmd sharing when uffd enabled

[Naresh Kamboju]

On Wed, 10 Mar 2021 at 22:27, Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> On Wed, Mar 10, 2021 at 01:18:42PM +0530, Naresh Kamboju wrote:
> > Hi Peter,
>
> Hi, Naresh,
>
> >
> > On Fri, 19 Feb 2021 at 04:43, Peter Xu <peterx@xxxxxxxxxx> wrote:
> > >
> > > Huge pmd sharing could bring problem to userfaultfd.  The thing is that
> > > userfaultfd is running its logic based on the special bits on page table
> > > entries, however the huge pmd sharing could potentially share page table
> > > entries for different address ranges.  That could cause issues on either:
> > >
> > >   - When sharing huge pmd page tables for an uffd write protected range, the
> > >     newly mapped huge pmd range will also be write protected unexpectedly, or,
> > >
> > >   - When we try to write protect a range of huge pmd shared range, we'll first
> > >     do huge_pmd_unshare() in hugetlb_change_protection(), however that also
> > >     means the UFFDIO_WRITEPROTECT could be silently skipped for the shared
> > >     region, which could lead to data loss.
> > >
> > > Since at it, a few other things are done altogether:
> > >
> > >   - Move want_pmd_share() from mm/hugetlb.c into linux/hugetlb.h, because
> > >     that's definitely something that arch code would like to use too
> > >
> > >   - ARM64 currently directly check against CONFIG_ARCH_WANT_HUGE_PMD_SHARE when
> > >     trying to share huge pmd.  Switch to the want_pmd_share() helper.
> > >
> > > Since at it, move vma_shareable() from huge_pmd_share() into want_pmd_share().
> > >
> > > Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> > > Reviewed-by: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
> > > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > > ---
> > >  arch/arm64/mm/hugetlbpage.c   |  3 +--
> > >  include/linux/hugetlb.h       |  2 ++
> > >  include/linux/userfaultfd_k.h |  9 +++++++++
> > >  mm/hugetlb.c                  | 20 ++++++++++++++------
> > >  4 files changed, 26 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/arm64/mm/hugetlbpage.c b/arch/arm64/mm/hugetlbpage.c
> > > index 6e3bcffe2837..58987a98e179 100644
> > > --- a/arch/arm64/mm/hugetlbpage.c
> > > +++ b/arch/arm64/mm/hugetlbpage.c
> > > @@ -284,8 +284,7 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
> > >                  */
> > >                 ptep = pte_alloc_map(mm, pmdp, addr);
> > >         } else if (sz == PMD_SIZE) {
> > > -               if (IS_ENABLED(CONFIG_ARCH_WANT_HUGE_PMD_SHARE) &&
> > > -                   pud_none(READ_ONCE(*pudp)))
> > > +               if (want_pmd_share(vma, addr) && pud_none(READ_ONCE(*pudp)))
> >
> > While building Linux next 20210310 tag for arm64 architecture with
> >
> >   - CONFIG_ARM64_64K_PAGES=y
> >
> > enabled the build failed due to below errors / warnings
> >
> > make --silent --keep-going --jobs=8
> > O=/home/tuxbuild/.cache/tuxmake/builds/1/tmp ARCH=arm64
> > CROSS_COMPILE=aarch64-linux-gnu- 'CC=sccache aarch64-linux-gnu-gcc'
> > 'HOSTCC=sccache gcc'
> > aarch64-linux-gnu-ld: Unexpected GOT/PLT entries detected!
> > aarch64-linux-gnu-ld: Unexpected run-time procedure linkages detected!
> > aarch64-linux-gnu-ld: arch/arm64/mm/hugetlbpage.o: in function `huge_pte_alloc':
> > hugetlbpage.c:(.text+0x7d8): undefined reference to `want_pmd_share'
> >
> > Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
>
> Sorry for the issue & thanks for the report.  Would you please check whether
> the patch attached could fix the issue?

The attached patch build tested and build pass for arm64
including 64k pages config.

 CONFIG_ARM64_64K_PAGES=y

- Naresh